The present invention relates to a dual-band antenna system of the beam waveguide type which is capable of varying the elevation and azimuth angles without limiting the settings of communications equipment or a transmitter/receiver.
A predominant type of large size antenna used for an earth station in a satellite communications system is the Cassegrain antenna, i.e., a dual reflector antenna having a main reflector and a subreflector. Associated with this type of antenna is a beam waveguide supply system which facilitates maintenance work and operation of communications equipment connected to the antenna, regardless of the rotatable antenna structure.
Prior art antenna systems employing such a beam waveguide supply system include those described in U.S. Pat. No. 3,845,483 (reference 1) assigned to NEC Corporation and issued Oct. 29, 1974, and U.S. Pat. No. 4,260,993 (reference 2) assigned to Thomson-CSF and issued Apr. 7, 1981.
The antenna system disclosed in reference 1 comprises at least a main reflector, a subreflector, two plane mirrors, two concave mirrors and an electromagnetic horn, as will be described. A drawback has existed in this type of antenna system in that, in feeding electromagnetic waves of dual (higher and lower) frequency bands (for example, 4 to 6 GHz and 11 to 14 GHz) to the antenna, the scope of design choice is limited because it is difficult to design and adjust a diplexer connected to the horn and adapted for the separation of the two frequency bands.
The antenna system of reference 2 is an attempt to overcome the drawbacks discussed above and employs another electromagnetic horn, a frequency selective reflector surface (referred to as a FSRS hereinafter) and three concave mirrors. The antenna system, as will be discussed in detail, includes two concave mirrors located in an electromagnetic path which leads from the horn allocated to one frequency band to the FSRS. While an electromagnetic path associated with the other frequency band has a single concave mirror therein, another concave mirror has to be furnished within this path so that the electrical characteristic of the antenna may not be effected by the rotation of the antenna in the azimuthal direction and thereby insure desirable cross polarization discrimination. Such a construction would naturally increase the number of concave mirrors in the system. Also, the propagation characteristics in the dual frequency bands are mutually different due to the difference in the surface accuracy between the two concave mirrors for the higher frequency band and those for the lower frequency band. This deteriorates the cross polarization discrimination.